The present invention relates to low voltage vacuum switches which are typically used in electrolytic chemical processing plants. The switches are used as shorting switches for shunting around a single series-connected electrolytic cell. Numerous such electrolytic cells are connected electrically in series to a D.C. plant power supply which provides several thousand amperes continuous current and an overall supply voltage of up to about 400 volts D.C. Since these cells are series-connected, and there may be up to about 100 such cells in series with approximately equal resistive characteristics, the voltage across an individual cell is about 4 volts D.C.
The vacuum switches are shunting switches for bypassing an individual cell without shutting down the plant. When a vacuum switch is in the closed contact position, effectively shunting the cell across which it is connected, about 4,000 amperes of D.C. current will be carried continuously through the switch, with the typical 4 volt D.C. potential across the switch. The shunted cell can then be electrically isolated from the system to permit inspection and maintenance. When the cell is to be put back into operation the vacuum switch is opened by moving the contacts apart. A high current, low voltage D.C. arc will burn for a short period within the vacuum switch. This arc will be extinguished when the contacts are sufficiently separated since an arc cannot be maintained in the vacuum switch when the potential across the switch contacts is below a characteristic switch contact arc voltage. For most materials this is about 15-20 volts D.C.
The high current arc which burns within the vacuum switch is typically extinguished in about twenty milliseconds. The contact material can be selected from any of the well known high conductivity materials such as high conductivity copper or copper-alloys which have low contact resistance in the closed switch position, and low weld strength for ease of switch opening. When the high current arc burns during switch openings, contact material is vaporized from the contact surfaces. Some of this vaporized contact material will condense on the interior surface of the switch envelope.
A low voltage vacuum shorting switch is described in application Ser. No. 650,322, filed Jan. 19, 1976 now abandoned in favor of continuing application Ser. No. 915,324 filed June 13, 1978. This switch comprises a pair of flexible annular corrugated diaphragms which are sealed to an annular insulating ring-like body at the diaphragm outer perimeters. A pair of opposed cylindrical conductive contacts are sealed through the inner perimeters of the respective diaphragms to complete the hermetically sealed switch envelope which is at least partially evacuated.
In copending application Ser. No. 928,640, filed July 27, 1978, this earlier low voltage vacuum switch was modified with an internal annular arc shield disposed within the switch between the contacts and the annular insulating body portion. This annular arc shield greatly reduced the deposition of vaporized contact material upon the annular insulating body portion of the switch which electrically isolates the switch ends when the contacts are in the open position. In one embodiment of the shielded low voltage switch a pair of overlapping members formed the arc shield. These overlapping members were annular and disposed concentric about the switch axis, but with the annular members directed parallel to the switch axis.
It has been discovered that after repeated operations some contact material does deposit behind the annular arc shield as evidenced by measuring the resistance along the annular ceramic insulating body. For a new switch, this resistance will be of the order of greater than one megohm. For a switch with an annular arc shield after about 50 switching operations the resistance across the annular insulator may be from several hundred to several thousand ohms.
It has been discovered that during certain unusual electrolytic cell conditions, when the vacuum shunting switch is in the open position, it is possible for the full plant power supply voltage, of up to about 400 volts D.C., to be applied across the switch. If there has been significant contact material deposited on the annular insulating body inner surface establishing a relatively low resistance path along the insulator, then a breakdown can occur across the vacuum switch. This can result in damage to the switch.